The development of systems of earth orbiting satellites, such as the Global Positioning System, capable of transmitting satellite orbital position and time with great accuracy has resulted in the further development of systems and methods for continuously or intermittently determining the geolocation of a device anywhere on the earth's surface or in an airborne vehicle thereabove.
A system and method have been developed which provides for autonomous operation of a device to determine the geolocation of the device which may be affixed to a vehicle, or hand carried by a person, and which is adapted to transmit the geolocation information by way of a LEO satellite to a remotely located receiving station, typically stationary on the earth's surface or possibly in an airborne vehicle, for example. Copending U.S. patent application Ser. No. 08/774,604 filed Dec. 30, 1996 and entitled: "Autonomous Geolocation and Message Communication System and Method" describes and claims several features of a system of the above-mentioned type and is assigned to the assignee of the present invention.
The invention described and claimed in the above-referenced patent application is operable to utilize signals transmitted by the satellite array or constellation of the Global Positioning System. The high earth orbit of the Global Positioning System (GPS) and the number of satellites deployed provides for communication of signals to a device in virtually any location on the face of the earth or in an airborne vehicle at any time to enable the device to determine its geolocation. However, the process of relaying geolocation information and other messages from the autonomous device to a remote receiving station by way of a selected LEO satellite can require additional considerations.
The particular LEO satellite selected for transmission of geolocation information to a remote information receiving station for monitoring or tracking the device will typically have certain visibility limitations, depending on the satellite orbital path and the position of the geolocation device transmitter at the time of desired information transmission. In order to minimize delays in transmitting data from the device to the receiving station, the relay LEO satellite must have covisibility with the transmitter and the receiving station to be able to relay the geolocation information without delay and without requiring storage of the information in suitable circuits on the satellite itself. Many LEO satellites may, of course, not have any capability for storing such information. Accordingly, covisibility between a particular LEO satellite, the autonomous device and the receiving station is mandatory.
Other considerations which must be taken into account when deploying a system as described in the above-referenced patent application include deployment constraints on the geolocation device including its transmitter. Depending on the geolocation transmitter position, certain obstacles, such as natural or manmade structures or electrical or magnetic fields, for example, may block "visibility" between the LEO satellite and the geolocation transmitter. Accordingly, it is important for the transmitter to know when the LEO satellite to which it is transmitting will be truly visible for signal transmission so as to avoid signal interruptions and resulting in incomplete information being transmitted to the receiving station. If the geolocation device transmitter is disposed on a vehicle, the obstructions which would limit visibility between the transmitter and a LEO satellite can also be dependent on the vehicle course or heading.
Accordingly, the present invention contemplates a geolocation communication system and method which takes into account the limitations on visibility of a receiver for geolocation information being transmitted from a geolocation communications device, and controls operation of the device to avoid transmission of signals during periods when such transmissions may be blocked before reaching the relay satellite or the information receiving station.